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Ricordarsi di non installare il pacchetto dnsmasq ma solo dnsmasq-base, perché il primo va in conflitto con Network-manager.
Extracted from link https://help.ubuntu.com/community/Dnsmasq
Note that the package “dnsmasq” interferes with Network Manager which can use “dnsmasq-base” to provide DHCP services when sharing an internet connection. Therefore, if you use network manager (fine in simple set-ups only), then install dnsmasq-base, but not dnsmasq. If you have a more complicated set-up, uninstall network manager, use dnsmasq, or similar software (bind9, dhcpd, etc), and configure things by hand.
Con Gnome è sempliccissimo: il pacchetto dnsmasq-base è già installato, basta aprire Gnome-nettools e configurare l'interfaccia eth1 in ipv4 come “partagé avec d'autres ordinateurs”.